In the production of decorative laminates, laminate assemblies composed of thermosetting phenolic resin impregnated Kraft paper core sheets, a thermosetting melamine resin impregnated decor sheet and, optionally, a thermosetting melamine resin impregnated overlay sheet are consolidated into press packs and laminated by the application of heat and pressure between the platens of a hydraulic press heated by steam, hot water or other suitable heating medium. A modern laminating press may have as many as twenty-two openings for sheet sizes up to 5 ft..times.12 ft. Since each opening can generally accommodate at least a ten-sheet pack, many laminates may be produced in each press cycle. When the pack has been placed between the press platen, pressure is applied until a net pressure in excess of 1,000 psi exists over the projected area of the sheets being laminated. With the pack under suitable pressure, the press temperature is raised by means of the fluid heating media which is introduced into the channels within the individual press platens. On a typical cycle, the temperature will rise to about 140.degree. C. within 30 minutes, remain at this level for 10 minutes to 30 minutes to accomplish curing of the resins, and return to room temperature in another 30 minutes. Cooling is accomplished by passing a cooling fluid through the same channels which guide the heating media or by simply cooling the heating media via a heat exchanger.
The word "curing" is a term frequently misused in the plastics industry. In molding and similar operations outside of the field of high-pressure laminates, curing refers to the transition of a resin from a soluble or fusible condition to an unsoluble-infusible condition by heat, chemical action or air drying. Thus when a liquid epoxy resin, for example, is poured into a mold and allowed to harden, it is said to be cured. In the manufacture of high-pressure laminates, curing takes place under heat and pressure in the platen press, as described hereinabove. However, after the fusion of the resin is accomplished, and the resin has become thermoset, there is an additional type of curing required which is more analogous to the annealing of glass. The laminate cannot be immediately removed from the press and allowed to cool rapidly in air because embrittlement of the surface layers will occur usually accompanied by warpage or buckling. As with newly cast or molded glass, the laminates must be slowly cooled by a gradual reduction of the heat and pressure to ambient and atmospheric conditions by gradually lowering the temperature of the platens.
After the curing is complete, the packs are removed from the press, the packs are disassembled and the laminates are sent on for finishing, while the press plates are returned for the next press run.
The water remaining in the press after the laminates have been removed is "blown out" with saturated steam during the first three to five minutes of the press cycle next accomplished. The temperature and flow rates through the press are then controlled to maintain the required time-temperature.
This means of press control has caused (1) an increase in the cost because of the expenditure of substantial amounts of energy, (2) an increase in the production of non-uniform products because of temperature variations in the press, (3) increased wear and tear on the apparatus components and (4) caused condensate and contaminate blocking of the press piping.